Kind of the core idea of this library. Think of it as a piece of data that described how to read and transform JSON. You can use decodeString and decodeValue to actually execute a decoder on JSON.

Decode a string.

""" "hello world" """
    |> decodeString string
--> Success "hello world"




""" 123 """
    |> decodeString string
--> Errors (Nonempty (Failure "Expected a string" (Encode.int 123)) [])

Decode a boolean value.

""" [ true, false ] """
    |> decodeString (list bool)
--> Success [ True, False ]

Decode a number into an Int.

""" 123 """
    |> decodeString int
--> Success 123




""" 0.1 """
    |> decodeString int
--> Errors <|
-->   Nonempty
-->     (Failure "Expected an integer number" (Encode.float 0.1))
-->     []

Decode a number into a Float.

""" 12.34 """
    |> decodeString float
--> Success 12.34




""" 12 """
    |> decodeString float
--> Success 12




""" null """
    |> decodeString float
--> Errors (Nonempty (Failure "Expected a number" Encode.null) [])

Decodes successfully and wraps with a Just. If the values is null succeeds with Nothing.

""" [ { "foo": "bar" }, { "foo": null } ] """
    |> decodeString (list <| field "foo" <| nullable string)
--> Success [ Just "bar", Nothing ]

Decode a list of values, decoding each entry with the provided decoder.

""" [ "foo", "bar" ] """
    |> decodeString (list string)
--> Success [ "foo", "bar" ]




""" [ "foo", null ] """
    |> decodeString (list string)
--> Errors <|
-->   Nonempty
-->     (BadIndex 1 <|
-->       Nonempty (Failure "Expected a string" Encode.null) []
-->     )
-->     []

Convenience function. Decode a JSON array into an Elm Array.

Convenience function. Decode a JSON object into an Elm Dict String.

Decode a JSON object into a list of key-value pairs. The decoder you provide will be used to decode the values.

""" { "foo": "bar", "hello": "world" } """
    |> decodeString (keyValuePairs string)
--> Success [ ( "foo", "bar" ), ( "hello", "world" ) ]

Decode the content of a field using a provided decoder.

""" { "foo": "bar" } """
    |> decodeString (field "foo" string)
--> Success "bar"




""" [ { "foo": "bar" }, { "foo": "baz", "hello": "world" } ] """
    |> decodeString (list (field "foo" string))
--> WithWarnings expectedWarnings [ "bar", "baz" ]




expectedWarnings : Warnings
expectedWarnings =
    UnusedValue (Encode.string "world")
        |> Nonempty.fromElement
        |> InField "hello"
        |> Nonempty.fromElement
        |> AtIndex 1
        |> Nonempty.fromElement

Decodes a value at a certain path, using a provided decoder. Essentially, writing at [ "a", "b", "c" ] string is sugar over writing field "a" (field "b" (field "c" string))}.

""" { "a": { "b": { "c": "hi there" } } } """
    |> decodeString (at [ "a", "b", "c" ] string)
--> Success "hi there"

Decode a specific index using a specified Decoder.

""" [ "hello", 123 ] """
    |> decodeString (map2 (,) (index 0 string) (index 1 int))
--> Success ( "hello", 123 )




""" [ "hello", "there" ] """
    |> decodeString (index 1 string)
--> WithWarnings
-->   (Nonempty (AtIndex 0 <| Nonempty (UnusedValue (Encode.string "hello")) []) [])
-->   "there"

Decodes successfully and wraps with a Just, handling failure by succeeding with Nothing.

""" [ "foo", 12 ] """
    |> decodeString (list <| maybe string)
--> WithWarnings expectedWarnings [ Just "foo", Nothing ]




expectedWarnings : Warnings
expectedWarnings =
    UnusedValue (Encode.int 12)
        |> Nonempty.fromElement
        |> AtIndex 1
        |> Nonempty.fromElement

Tries a bunch of decoders. The first one to not fail will be the one used.

If all fail, the errors are collected into a BadOneOf.

""" [ 12, "whatever" ] """
    |> decodeString (list <| oneOf [ map toString int, string ])
--> Success [ "12", "whatever" ]




""" null """
    |> decodeString (oneOf [ string, map toString int ])
--> Errors <| Nonempty.fromElement <| BadOneOf
-->   [ Nonempty.fromElement <| Failure "Expected a string" Encode.null
-->   , Nonempty.fromElement <| Failure "Expected an integer number" Encode.null
-->   ]

Decode a JSON string. If the string isn't valid JSON, this will fail with a BadJson result.

Run a Decoder on a Value.

Note that this may still fail with a BadJson if there are non-JSON compatible values in the provided Value. In particular, don't attempt to use this library when decoding Events - it will blow up. Badly.

Decoding can have 4 different outcomes:

• BadJson occurs when the JSON string isn't valid JSON, or the Value contains non-JSON primitives like functions.
• Errors means errors occurred while running your decoder and contains the Errors that occurred.
• WithWarnings means decoding succeeded but produced one or more Warnings.
• Success is the best possible outcome: All went well!

A simple type alias for Json.Decode.Value.

Decoding may fail with 1 or more errors, so Errors is a Nonempty of errors.

The most basic kind of an Error is Failure, which comes annotated with a string describing the failure, and the JSON Value that was encountered instead.

The other cases describe the "path" to where the error occurred.

Decoding may generate warnings. In case the result is a WithWarnings, you will have 1 or more warnings, as a Nonempty list.

Like with errors, the most basic warning is an unused value. The other cases describe the path to the warnings.

Useful for transforming decoders.

""" "foo" """
    |> decodeString (map String.toUpper string)
--> Success "FOO"

Combine 2 decoders.

Combine 3 decoders.

Combine 4 decoders.

Combine 5 decoders.

Combine 6 decoders.

Combine 7 decoders.

Combine 8 decoders.

Decode an argument and provide it to a function in a decoder.

decoder : Decoder String
decoder =
    succeed (String.repeat)
        |> andMap (field "count" int)
        |> andMap (field "val" string)




""" { "val": "hi", "count": 3 } """
    |> decodeString decoder
--> Success "hihihi"

Required when using (mutually) recursive decoders.

Extract a piece without actually decoding it.

If a structure is decoded as a value, everything in the structure will be considered as having been used and will not appear in UnusedValue warnings.

""" [ 123, "world" ] """
    |> decodeString value
--> Success (Encode.list [ Encode.int 123, Encode.string "world" ])

Decode a null and succeed with some value.

""" null """
    |> decodeString (null "it was null")
--> Success "it was null"

Note that undefined and null are not the same thing. This cannot be used to verify that a field is missing, only that it is explicitly set to null.

""" { "foo": null } """
    |> decodeString (field "foo" (null ()))
--> Success ()




""" { } """
    |> decodeString (field "foo" (null ()))
--> Errors <|
-->   Nonempty
-->     (Failure "Expected an object with a field 'foo'" (Encode.object []))
-->     []

A decoder that will ignore the actual JSON and succeed with the provided value. Note that this may still fail when dealing with an invalid JSON string.

If a value in the JSON ends up being ignored because of this, this will cause a warning.

""" null """
    |> decodeString (value |> andThen (\_ -> succeed "hello world"))
--> Success "hello world"




""" null """
    |> decodeString (succeed "hello world")
--> WithWarnings
-->     (Nonempty (UnusedValue Encode.null) [])
-->     "hello world"




""" foo """
    |> decodeString (succeed "hello world")
--> BadJson

Ignore the json and fail with a provided message.

""" "hello" """
    |> decodeString (fail "failure")
--> Errors (Nonempty (Failure "failure" (Encode.string "hello")) [])

Chain decoders where one decoder depends on the value of another decoder.